Chromium-nitrogen die steel



United States Patent Office 3,105,758 Patented Oct. 1, 1963 3,105,758 CHRGMIUlvI-NITROGEN DIE STEEL Joseph Field, Bethlehem, Pa., assignor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing. Filed .l'an. 9, 1961, Ser. No. 81,267 6 Claims. (Cl. 75-128) This invention relates to an improved hot work steel.

More specifically it relates to an improved hot work steel that is adapted particularly to dies for die casting of metals such as aluminum and zinc and therefore may be described with particular reference thereto.

Steels used for dies and die parts in the die casting industry must possess high hardness or strength, resistance to heat corrosion, and of particular importance, high resistance to heat checking. This is due, to a large extent, to the long periodsv of time that the dies are in contact with the hot metal castings. With the removal of the castings from the dies, the surfaces of the dies are exposed to cooling. Thus the die surface is repeatedly exposed to a broad range of temperature reversals which results in heat checking, the main cause of die failure.

Die steels usually used for the purposes of die casting aluminum and zinc are conventionally made of hot work tool steel compositions designated as AISI type H13 of approximately .35 carbon, 5.00% chromium, 1.00% vanadium, 1.50% molybdenum, and with no nitrogen addition. The steels of the prior art have the desired hardness at operating temperatures but are relatively susceptible to heat checking. Therefore, the service life of dies of these compositions is measured by the length of time required to develop heat checks. The dies may have to be discarded or repaired, depending on the severity of these heat checks. It is generally believed that heat checking results from the stresses developed by the repeated heating and cooling of the die surfaces in service. In this connection the presence of brittle grain boundary carbides promotes heat checking. Similarly there is reason for believing that heat checking may be accelerated as a result of corrosion attack which occurs at the elevated temperatures.

An object of this invention is to provide hot work tool steel compositions that possess primarily an improved resistance to heat checking in conjunction with high hardness and strength at the die operating temperatures.

The hot work tool steel compositions of my invention contain nitrogen, together with lower percentages of carbon and higher percentages of chromium than found in the conventional hot work tool steels usually used for die casting of aluminum and zinc.

The steels of my invention contain:

Preferred Broad Range Range, percent C .20/.30% 20/.25. Mn. 1.00% max 45/.75. Si. .75/L557 LOO/1.50 CL-.. sou/13.00% 900/1200 V--- .85/1.15% .90/1.10 Mo .95/1.45% LOO/1.40 W .30/.60% .35/.55. N .05% Min .07/.12.

specific analysis of a typical heat of steel embodying this invention is as follows: carbon 24%, manganese .60%, phosphorus 010%, sulphur 007%, silicon 1.50%, nickel .76%, chromium 10.16%, vanadium 99%, molybdenum 1.32%, tungsten :44%, nitrogen 07%, and the balance iron except for minor impurities. From this heat six 12" ingots were hammer forged to 5" x 5 square billets, then rolled to 2 /2" diameter bars, and subsequently rolled to 1% diameter bars. Test data determined on these bars have revealed that steels of my new compositions can be successfully hardened to 52/ 53 Rockwell C by air quenching from 1850 A1900? F. and subsequently tempered at 1025 /1050 F. to provide a hardness of 45/48 Rockwell C, which is considered desirable for die applications. These hardnesses have been attained at no sacrifice of other properties such as ductility .and impact resistance.

Double-tempering, is a generally accepted practice in the heat treatment of these types of tool steels. Therefore, it is advisable with my new composition.

The data are shown in the following tables:

ROOM TEMPERATURE PROPERTIES [Quenehed from 1900 F. and double tempered] HIGH TEMPERATURE PROPERTIES 7 [Quenched from 1900 F.D0uble tempered at 1025 F. (Re 46/47)] Test Temp, F. Y.S.(.2%) '1.S. Elon. Red.

(psi) (p.s.i.) (percent) (percent) The above'd-ata show that my new'compositions can be treated to "a high strength level with good ductility and impact resistance, and the high temperature tests show that? these steels possess a favorable combination of strength and ductility at elevated temperatures.

Res'istanceto heat checking in dies used for die casting of metals is a physical property of steels that cannot be determined satisfactorily in the laboratory. Therefore, this property, the chief cause of failure, can best be determined by actual service life.

In die casting operations, molten metal is forced or led to the die proper through orifices commonly known as gates. The material used for gates must be suitable to resist heat checking, a condition which may ultimately lead to failure of this part. In order to investigate the advantages of my new die steel with respect to its resistance to heat checking, center post gates made of AISI type H13 steel and of the new steel of my invention were tested in service in the die casting of aluminum. For this comparison the aluminum metal removed from the gates was examined, since any heat checking of the gate material manifests itself by appearing as cracks on the aluminum material. It was noted that after three months service and 200,000 shots of molten aluminum, heat check cracks were apparent on the gate made of conventional steel whereas the gate made of my new steel has given twoyears service and 2,000,000 shots without any evidence of heat checking.

From the foregoing, it will be noted that my new steel achieves the object of my invention, i.e. to provide a hot work tool steel that possesses an improved resistance to heat checking.

Although I have described a specific composition and disclosed a preferred application for the composition, it will be understood that these are merely embodiments and do not limit the scope of the invention relative to other applications requiring hot work tool steels possessing the properties of high strength with good ductility, resistance to heat, and resistance to heat checking.

I claim:

1. Alloy steel having resistance to heat checking and corrosion, said steel consisting essentially of the following elements by Weight:

2. A hot work die steel characterized by resistance to heat checking and corrosion when subjected to repeated temperature variations at elevated temperatures, said steel consisting essentially of the following elements by Weight:

Percent Carbon .20/ .25 Manganese .45/ .75 Silicon 1.00/ 1.50 Nickel .65/.85 Chromium 9.00/ 11.00

Vanadium .90/l.10 Molybdenum LOO/1.40 Tungsten .35 .55 Nitrogen .07 .12 Iron Balance 3. A hot work die steel characterized by resistance to heat checking and corrosion, said steel. consisting essentially :of the following elements by weight:

consisting essentially of p 7 Percent Carbon -a .20/.25 Silicon LOO/1.50 Chromium 9.00/ 11.00 Vanadium u .90/ 1.10 Molybdenum 1.00/ 1.40 Tungsten .35/.55 Nitrogen -Q. .07/.l2 Iron Balance 5. A die casting die formed from a steel having a Rockwell C hardness of 45/48 and characterized by resistance to formation of heat checks, said steel consisting essentially of the following elements by weight:

Carbon .20/.'30% Manganese 1.00% maximum Silicon .75/l.55% Nickel .55/.95% Chromium 8.00/13.00%

' Vanadium .85/l.15%

Molybdenum .95/ 1.45 Tungsten .30/.60%

' Nitrogen .05/.12% rlron Balance 6. A die casting die having resistance to the formation of heat checks consisting essentially of the following elements by Weight: 7

Carbon 7 20/.30 Manganese 100% maximum Silicon .75/1.55% Nickel V .55/.95% Chromium 8.00/13.00% Vanadium .8S/l.15% Molybdenum -Q .95 1.45 Tungsten 037.60% Nitrogen .05/.l2% Iron Balance 7 References Cited in the fileof this patent UNITED STATES PATENTS 2,147,123 Emmons Feb. 14, 1939 2,853,410 Lula' Sept. 23, 1958 Percent Carbon .20/ .30 Silicon .75 1.55 Chromium 8.00/ 13.00

Vanadium ,85/1.15 Molybdenum .95/ 1.45 Tungsten .30/ .60

Nitrogen v Iron Balance 4. A die casting die steel characterized by resistance to heat checking in the hardened condition, said steel UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 105 758 0c tober l U 1963 Joseph Field It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 5 after "consisting essentially of" insert the following elem ents by weight line 41 for "Tungsten .O3/.60%" read Tungsten .30/. 60%

Signed and sealed this 19th day of May 1964.

(SEAL) Attest: ERNEST W. SWIDER EDWARD J BRENNER Attesting Officer Commissioner of Patents 

1. ALLOY STEEL HAVING RESISTANCE TO HEAT CHECKING AND CORROSION, SAID STEEL CONSISTING ESSENTIALLY OF THE FOLLOWING ELEMENTS BY WEIGHT: CARBON .20/30% MANGANESE 1.00% MAXIMUM SILICON .75/1.55% NICKEL .55/95% CHROMIUM 8.00/13.00% VANADIUM .85/1.15% MOLYBDENUM .95/1.45% TUNGSTEN .30/.60% NITROGEN .05/.12% IRON BALANCE 